v0.2.3

This new minor release corrects a lot of things suggested on Reddit - thanks!

✨ Additions

• Error handling for JSON parsing

🔨 Changes

• All the exceptions are now based on a single OpenWeatherMap::Exception class
• lang and unit attributes of the API are now read-only, to prevent bypassing the check
• Made all the documentation compatible for RDoc
• BREAKING CHANGE : the default unit for the API is default rather than just nil, that suggested "don't use unit system"

See #2 for more informations.